Anycast is a network addressing and routing methodology whereby the same network addressing is advertised from two or more different network locations. FIG. 1 conceptually illustrates an Anycast based network architecture. There are two different network locations, also referred to as points-of-presence (PoPs) 110 and 120, from which one or more servers operate to deliver the same content or services to different requesting clients. Each PoP 110 and 120 advertises that it is accessible at the same Internet Protocol (IP) address. In particular, the gateway router at each PoP 110 and 120 distributes Border Gateway Protocol (BGP) advertisement messages (i.e., BGP update messages) to neighboring network routers. The advertisements convey the number of hops to an address through a sequence of Autonomous System (AS) identifiers or other identifiers. The advertisement messages identify the same IP addressing for reaching each PoP 110 and 120.
The other network routers receive the advertisements and build routing or forwarding tables that identify the shortest path to each advertised address. Thus, when a request directed to the Anycast address advertised from PoPs 110 and 120 is received by router 130, router 130 identifies PoP 110 as the closest destination and routes the request to PoP 110. When a request directed to the Anycast address advertised from PoPs 110 and 120 is received by router 140, router 140 identifies PoP 120 as the closest destination and routes the request to PoP 120 instead of PoP 110.
Anycast provides various advantages over traditional Unicast addressing and routing. For Unicast, each destination is assigned a unique address. Domain Name System (DNS) query resolution is complicated by the fact that if two different locations or PoPs host content for the queried domain, the DNS server performs some logic to identify the optimal location before resolving the query. No such logic is required for Anycast because each query to a particular domain that is accessible at a particular Anycast address is simply resolved to that particular Anycast address regardless of where the query is received, where the query originated from, or where a subsequent request is to be routed to. For Unicast, the logic typically involves the DNS server looking up its own location or the location of the requestor in order to identify the Unicast address of the closest PoP or server for the queried domain. This is one example in which Anycast can provide improved performance relative to Unicast.
An Anycast based network architecture is also significantly easier to setup and manage than a Unicast based network architecture. There are fewer addresses to manage and link to geographic regions or other criteria, thereby reducing the likelihood for improper or suboptimal routing of requests. There is also significantly less configuration overhead. In a Unicast based network architecture, every time DNS operation is changed to route a domain from a first Unicast address to a second or other Unicast address, that change is tracked so that the DNS operation can be restored or reset if needed.
Anycast involves relinquishing control of the routing to the network. As a result, Anycast does not allow for deterministic shifting of a specific subset or a specific amount of traffic from one Anycast location to other specifically selected Anycast locations. Such deterministic shifting of traffic is easily achieved in a Unicast based network architecture by simply changing the DNS resolution for the traffic that is to be shifted from the first address of a source location to the different second address of a desired destination location. However, since all locations in an Anycast based network architecture advertise the same addressing, such deterministic traffic shifting via DNS resolution is not possible. Pulling the address advertisements from the source location is one manner to shift traffic in the Anycast based network architecture, but the amount of traffic or load to be shifted from the source location and which destination locations that traffic is shifted to is not deterministically controllable by simply pulling the address advertisements.
Deterministic management of traffic is effective in combatting network attacks, maintaining uptime in the event of an outage or failure in a particular location, balancing capacity in response to high demand or usage at a particular location, and improving performance. There is therefore a need to provide deterministic traffic controls in an Anycast based network architecture. In particular, there is a need to retain the simplicity and performance advantages of Anycast while also having the ability to shift specific amounts of traffic to specifically selected destinations within the Anycast based network architecture.